Hardest problem this semester

With the semester winding down, I thought I would ask folks what the hardest problem they were assigned this semester was. It can be from any physics class, but please specify the class in your post. Also, let us know if you figured it out, and how you did it. After a few posts, I'll tell you the hardest problem I assigned.

Dave

Try not to become a man of success, but rather try to become a man of value -- Albert Einstein